The water changes are the reason. Mine grew faster than yours, because I cleaned it every day. With 40 or so in a 10, they will take longer to grow because of the growth stunting hormone and ammonia build up from the foods, and the fish's waste. Every day would be best - even twice a day to get them to literally double in size.
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.